This invention relates to a locking device for a hydraulic cylinder, particularly a hydraulic cylinder which must be securely locked in either the fully-extended position, or the fully-retracted position. Typical hydraulic cylinder locking devices, to date, have employed either a strictly mechanical locking arrangement, a strictly hydraulic locking arrangement, or where a combined mechanical/hydraulic device has been provided, the mechanical and hydraulic portions were not used in a cooperative manner. In the case of the mechanical approach, certain components are exposed to extreme straining forces, resulting in excessive wear on those components. Generally, such high wear components comprise a spring-biased plunger movable over a cam portion formed on the piston rod whereby the spring, plunger, and cam portions require frequent maintenance and replacement. Another mechanical approach employs a clamping arrangement to the piston rod, such clamping arrangement also experiences the same wear and high stress problems previously discussed. In the case of the hydraulic approach, lock slippage due to leakage, or "give" of the seals, has been the most common problem, such lock slippage resulting in the cylinder rod drifting from the original position.
Additionally, such hydraulic cylinder locking arrangements have been restricted as to the load range over which an individual locking device could operate. Typically, as the load requirement changed, it was necessary to substitute a different component such as, for example, a differnt-sized plunger.